A working machine, such as a construction machine or the like, typically has a number of different hydraulically powered functions that are controlled by a closed center hydraulic system. Such hydraulic systems typically feature more than one engine driven, variable displacement hydraulic pump, the displacement of which is responsive to the demands of the system.
On mobile applications an internal combustion engine (ICE) typically drives an electronically or mechanically controlled hydraulic pump, which is used to power hydraulic components. Generally, the engine speed is manually set by an operator or controller programmer and the torque is adjusted by the displacement of the pump according to the power demand of the hydraulic system. As the engine power output moves along a vertical line of constant speed the efficiency of the engine changes dramatically, as generally low torque areas of the engine equate to low efficiency and high torque areas equate to high efficiency.
Techniques are known for changing the power output of an engine according to the work load. According to such techniques, the working machine may include a “heavy work mode” in which it is possible to obtain a high (e.g., maximum) power output, and a “light work mode” in which it is possible to obtain power output up to a predetermined (low) level.
In operation, the driver manually selects between these modes via a changeover switch. In other words, if the operator determines that the work that will be performed henceforward is heavy work, then he selects the heavy work mode, while if he determines that it is light work, then he selects the light work mode.
Based on the selected mode, a controller for controlling the engine controls the power output capacity of the engine. For the light work mode the controller limits the power output of the engine to be less than or equal to a predetermined value (which is lower than the rated engine power output), for example, by limiting the amount of fuel supplied to the engine. In the heavy work mode, the controller does not impose a limitation on the fuel and therefore it is possible for the power output of the engine to reach the rated power.
As will be appreciated, often times the working machine does not continuously perform only heavy work or only light work, but rather alternates between heavy work and light work during a series of work processes. For example, an excavator, during typical excavation and loading, successively performs processes such as approaching a location (which is light work), excavating at the location and loading the excavated material into the bucket (which is heavy work), and dumping of the subject material for work upon a transportation vehicle such as a dump truck or the like (which is light work).
With such working processes, in order to enjoy the benefits of the prior art technique to the maximum level the driver must operate the changeover switch while changing over between heavy work and light work. However, it can be burdensome to perform such switch operation for each working process. As a result, it often happens that work is performed with the changeover switch maintained in the heavy work mode, thereby not taking advantage of the fuel savings afforded by the light work mode. On the other hand, if emphasis is placed upon fuel consumption and work is performed while in the light work mode, then it may not be possible to obtain sufficient power output during heavy work, and which can result in loss of efficiency.